Binding energy of singlet excitons and charge transfer complexes in MDMO‐PPV:PCBM solar cells

Abstract

In recent years, joint efforts of a vast and expanding research community have led to a steady improvement of organic solar cell (OSC) power conversion efficiencies. However, even OSC record efficiencies of currently 8.3% [1] are still trailing behind the ones of their inorganic counterparts -a fact which can be attributed to the relatively low dielectric constants r ( 3 4) ε ªof the employed organic semiconductors. The resulting weak screening causes the primary photoexcitations, i.e. the singlet excitons S 1 created in the polymer, to be strongly bound, thus making thermally induced dissociation to free charge carriers highly improbable. In the bulk heterojunction (BHJ) solar cell concept, dissociation is driven by ultrafast photoinduced charge transfer between a donor and an acceptor material (e.g. a conjugated polymer -fullerene system). The interpenetrated donor -acceptor network leads to the creation of charge transfer complexes (CTC), i.e. bound electron-hole pairs at the heterointerface forming after singlet exciton dissociation. While the energetics of those complexes are already known to influence the overall OSC efficiency by setting the maximum limit for the open circuit voltage [2], the extent of the CTC's impact on the equally important photocurrent is still controversially debated [3,4]. Nevertheless, there exists a common notion that CTC can be precursors to at least some of the generated free charge carriers if the coulombic CTC binding energy can be overcome by an additional driving force. In order to contribute to the understanding of the underlying physical processes, we report on the determination of the binding energy 1 b,S E of the singlet exciton in poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV) and b,CTC E of the CTC of the PPV derivative blended with [6,6]-phenyl-C 61 butyric acid methylester (PCBM).Due to the finite binding energy, a certain amount of the created singlet excitons or CTC is not dissociated but recombines. The radiative part of the recombination can be observed as photoluminescence (PL). The application of an increasing external electric field constitutes an additional driving force which leads to an enhanced singlet exciton or CTC dissociation and thus to a reduction of the PL intensity. This field dependent PL quenching (PL(F)) has been observed for both singlet excitons and CTC for a variety of material systems [5,6] and is also regarded as a suitable technique for determining the binding energy [5,7] by relating it to the established Onsager -Braun model [8,9]. This model comprises Braun's adaption of the Onsager theory to the case of the electron-hole pair being the lowest excited state of the observed system [10] and thus ap-The influence of an external electric field on the photoluminescence intensity of singlet excitons and charge transfer complexes is investigated for a poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV) diode and a bulk heterojunction of the PPV in combination with [6,6]-phenyl-C 61 butyric ac...

Direct and charge transfer state mediated photogeneration in polymer–fullerene bulk heterojunction solar cells

“…Here, we investigated organic BHJ solar cells containing blends of either MDMO-PPV:PC 61 BM or P3HT:PC 61 BM by temperature dependent TDCF measurements. We observed a strongly field dependent polaron pair dissociation in blends of MDMO-PPV:PC 61 BM, which is in good agreement with its CT exciton binding energy of up to 200 meV, determined by photoluminescence measurements [13,14]. In contrast, an ala Electronic address: deibel@physik.uni-wuerzburg.de most field and temperature independent charge collection was experimentally found in P3HT:PC 61 BM.…”

Polarization anisotropy of charge transfer absorption and emission of aligned polymer:fullerene blend films

“…Such CT emission bands have been detected previously for several polymer:fullerene systems, using PL and electroluminescence measurements. 9,10,[43][44][45] The polarization anisotropy r PL (E) spectra for films aligned at 80 • C (Fig. 2) and 110 • C are shown in Fig.…”

Revelation of Interfacial Energetics in Organic Multiheterojunctions

“…[41] Besides the above mentioned two-step charge generation process, constituted of an excitation of the donor material followed by a charge transfer to the acceptor or vice versa, lately a single step charge generation process via direct excitation of the CT-state located at the donor-acceptor interface has been discovered and discussed with growing intensity. [42][43][44][45][46][47] Early studies using highly sensitive external quantum efficiency measurements already revealed that below both the electronic transitions of neat donor and acceptor in the blend an additional very weak transition exists, representing the CT. [42][43][44][45][46][48][49][50][51][52][53][54] Furthermore it has been shown by electroluminescence (EL) measurements that also the opposite process, i.e., luminescent recombination via a CT-state, can be detected. [42][43][44][45]54,55] One major conclusion was that the transition energy of this CTstate is inherently linked to the energetic separation of charge carriers and thus to the open-circuit voltage of organic solar cells.…”